The invention concerns a method for testing a new superconducting wire through charging an actively shielded magnet coil configuration (20) comprising a first partial region (21) which can be superconductingly short-circuited using an additional switch (23). A superconductor is used in this first partial region (21), which is to be tested under increased current load without subjecting the superconductor in the second partial region (22) to increased current load. Both partial regions (21, 22) have associated operating currents, wherein the first partial region shows the desired excess current and the overall field B0 differs only slightly from the standard operating field of the magnet coil configuration. These operating currents are adjusted through initial charging of the overall magnet coil configuration (20), thereby taking into consideration the inductive coupling between the partial regions (21, 22), and after closing the additional switch (23), through continuation of charging or discharging processes only in the second partial region (22). In this manner, the superconductor can be tested in a series-produced NMR magnet under NMR conditions and in an inexpensive fashion.
One essential problem in the construction of a superconducting NMR (nuclear magnetic resonance) magnet with maximum temporal field stability is the proper loading of the superconducting wire in the magnet. The NMR magnet should be as small and compact as possible to reduce costs. In order to achieve this, the superconducting wire (superconductor) must be loaded with a maximum current. However, the current must not be excessive, since otherwise the resistance of the superconductor, which cannot be completely eliminated in an actual superconductor, and the associated current reduction in the magnet, i.e. the magnetic field drift, would become excessive. The magnetic field drift of NMR magnets is defined, as is the field strength itself, relative to the resonance frequency of the hydrogen nucleus (proton), which, in turn, is proportional to the magnetic field. The specific maximum admissible field drift of NMR magnets is on the order of magnitude of 0.01 ppm/h. This would be a specific drift of 8 Hz/h for an 800 MHz magnet. As can be shown by simple calculation, a superconducting NMR magnet having this specification cannot be operated at a current that corresponds to the critical current of the superconductor used. According to a generally accepted definition, the critical current is reached when the voltage drop across one centimeter of the superconductor is 0.1 pV. As the magnetic field in which the superconductor is located increases, the critical current decreases. With a conductor length of typically 100 km wound in a highest-field magnet, operation of a conductor length of 1 km in a magnetic field region in which the magnetic current corresponds to the critical current of the superconductor is sufficient to obtain a highly drifting magnet. The discharging voltage applied to the magnet would be 0.01 V in this case. The magnet current must obviously be considerably less than the critical current of the superconductor. If the magnet current is excessively low, the number of magnet windings required to achieve the required field would be excessively high, making it unacceptably large and expensive.
The determination of the correct magnet current is further aggravated in that the maximum voltage drop of the superconductor per centimeter that still prevents the overall magnet from drifting is sufficiently small that it can no longer be distinguished from the noise. The voltage drop across a superconducting magnet through which an operating current flows, and its magnetic field drift, cannot therefore be derived from a measurement on a short superconducting piece through which a magnetic flux flows. Maximum resolution voltage measurement over the entire conductor length wound on the coil body is also not possible.
In an alternative method, the superconductor to be tested is installed in an existing series-produced magnet. In this case, the conductor could be tested under NMR conditions, however, with two decisive limitations. First, the conductor dimension (conductor cross-section) of the series-produced magnet is used, which means that the new conductor type cannot bear a higher load than the standard conductor from the series-produced magnet. The higher potential of the new conductor can therefore not be tested. Secondly, if the conductor cross-section of the new conductor type is smaller, the conductor can carry a higher current density, however, the construction of the coil body of the series-produced magnet must be completely changed and adjusted to the new conductor dimension which would involve high constructive effort and expense. If the conductor does not meet the expectations and would be excessively loaded in the series-produced magnet, this magnet could not be delivered.
It is the underlying purpose of the present invention to test the NMR capability of a new superconductor type with NMR sensitivity in a series-produced NMR magnet without additional effort and little risk of having to replace the tested superconductor, i.e. having to repair the series-produced magnet. Towards this end, the new superconductor type to be tested, which should have e.g. a higher current carrying capacity than the standard superconductor, must have the same dimensions as the conductor being replaced in the series-produced magnet. The method allows testing of the new superconductor under increased current load or current density load compared to the series-produced magnet with simultaneous control of the magnetic field stability (magnetic field drift) at the NMR level. The superconductor test can be performed in a conventional series-produced magnet provided with an additional superconducting switch: the construction of the coil bodies remains unchanged since the conductor dimensions are the same. After the test, the magnet including new superconductor type can be delivered and operated according to specifications. Even though the new conductor type does not meet the increased expectations, the magnet may be delivered as long as the conductor is not worse than the standard conductor conventionally used in series-produced magnets.